A repeater is installed in an area where a signal from a main transmitter is received at a weak level, and it can solve an unstable reception and broaden a coverage area of the main transmitter.
Generally, channel distortion occurs between the main transmitter and the repeater. The repeater compensates for the distortion of a receiving channel between the main transmitter and the repeater and retransmits the compensated signal. To this end, the repeater estimates a receiving channel, generates a time-domain filter coefficient having information on an inverse channel of the estimated receiving channel, and compensates for the distortion of the receiving channel by filtering a received signal using the generated filter coefficient.
A conventional method for estimating an inverse channel of a repeater's receiving channel will be described below with reference to FIG. 1.
Referring to FIG. 1, an inverse channel (HINV) 12 is generated from an estimated receiving channel (H) 11 by a direct inverse transformation using division, which is expressed as the following Equation 1:
      MathFigure    ⁢                  ⁢    1                                                                                            H                  INV                                =                                                      [                                                                                            H                          INV                                                ⁡                                                  (                          0                          )                                                                    ⁢                                                                        H                          INV                                                ⁡                                                  (                          1                          )                                                                    ⁢                                                                                          ⁢                      …                      ⁢                                                                                          ⁢                                                                        H                          INV                                                ⁡                                                  (                                                      N                            -                            1                                                    )                                                                                      ]                                    T                                                                                                        =                                                      [                                                                  1                                                  H                          ⁡                                                      (                            0                            )                                                                                              ⁢                                              1                                                  H                          ⁡                                                      (                            1                            )                                                                                              ⁢                                                                                          ⁢                      …                      ⁢                                                                                          ⁢                                              1                                                  H                          ⁡                                                      (                                                          N                              -                              1                                                        )                                                                                                                ]                                    T                                                                                          [                      Math            .                                                  ⁢            1                    ]                    
where N represents the number of samples of the estimated channel, and T represents a transpose.
The inverse transformation using division can ensure stability and causality when z-plane's poles and zeros of the estimated receiving channel are located inside the unit circle as illustrated in FIG. 2.
However, in the real environment, a part of the z-plane's poles and zeros of the estimated receiving channel are located outside the unit circle as illustrated in FIG. 3. Therefore, in the real environment, the channel estimation using division cannot ensure stability and causality.